It is known that the prior art includes semidirect coal firing in a system which does not have an inert atmosphere and is therefore, vulnerable to fire and explosion. Such prior art systems are normally built at great expense to sustain internal pressures of significant magnitude to contain all by the most violent explosion. In these prior art systems the ambient air is conducted through a heater which merely heats the air without burning out some of the oxygen. The hot air at normal 21 percent oxygen is introduced into the coal grinder where it is intended to dry the moisture and function as the transporting media for the ground coal. After transporting the ground coal to a separator, the air is returned to the grinder, but part of the returning air by passes the grinder and is employed to move the separated ground coal into the furnace where it is consumed.
While the foregoing system cannot be classified as an inert atmosphere system, I have disclosed in my prior U.S. Pat. No. 3,477,650 of Nov. 11, 1969 apparatus to provide an inert atmosphere in a material reducing mill by utilizing the inert air in the system to mix with fresh air in a furnace where the mixture is used to support combustion to the extent that the flue gas is inerted and at a temperature suitable for drying the material during its reduction and exit from the reducing mill. Any dust fractions moved with the drying and inert media are removed before such media is admitted to the furnace.
An early system of using heated air for drying as well as transporting pulverized material is disclosed by Crites et al in U.S. Pat. No. 1,783,358 of Dec. 2, 1930, but the system made no attempt to reduce the oxygen level to establish an inert atmosphere, nor did it employ the pulverized material as the fuel for producing the drying heat.